The successful implementation of a light wave communication system requires high quality light guide fibers having mechanical properties sufficient to withstand the stresses to which they are subjected. Each fiber must be capable of withstanding over its entire length a maximum stress level to which the fiber will be exposed during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundreds of circuits.
The failure of light guide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength below that of pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress. Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Optical fibers are normally made in a continuous process which involves drawing a thin glass strand of fiber from a partially molten glass preform and thereafter applying the coating layers. A furnace is used to partially melt the preform to permit the fiber to be drawn. The heat of the furnace and the rate of draw of the fiber must be in proper balance so that the optical fiber can be drawn continuously under uniform conditions. Long lengths of light guide fibers have considerable potential strength, but the strength is dimnished by airlines or holes occurring in the optical fibers. Furthermore, airlines in optical fibers also interfere with the light-propagation properties of the optical fibers.
Soon after an optical fiber is drawn, the optical fiber is coated with a layer of a coating material such as, for example, a polymer. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber, which would weaken it or even affect its transmission properties. Also, the coating shields the fibers from surface abrasion, which could occur as a result of subsequent manufacturing processes and handling during installation. The coating also provides protection from corrosive environments and spaces the fibers in cable structures. However, this coating layer does not eliminate problems caused by airlines or holes existing in the fiber itself. The above-referenced co-pending related applications, Ser. Nos. 08/815,180 and 08/814,673, are directed to detecting defects in an optical fiber coating and detecting and distinguishing between defects in an optical fiber coating, respectively.
It is generally known in the industry to monitor optical fibers as they are being drawn during the manufacturing process to determine whether defects exist in the optical fibers. However, the known techniques analyze the optical fibers during the drawing process before the coating layers have been applied and require complicated hardware and/or software to detect defects contained in the optical fibers.
For example, Bondybey et al., U.S. Pat. No. 4,021,217, disclose a system for detecting optical fiber defects to determine the tensile strength of optical fibers as they are being manufactured prior to any coating layers being applied to the optical fiber. The apparatus disclosed in the Bondybey et al. patent projects a focused beam of monochromatic light onto an optical fiber as it is being drawn. A photodetector, such as a photomultiplier, is positioned off axis with respect to the direction in which the light is projected onto the optical fiber so that it receives only scattered light unique to defects contained in the fiber. The output of the detector is received by an electrometer strip chart recorder which plots a scattering trace corresponding to the light detected. The peaks in the scattering trace correspond to defects in the optical fiber.
Button et al., U.S. Pat. No. 5,185,636, disclose a method for detecting defects such as holes in a fiber. The apparatus disclosed in the Button et al. patent utilizes a laser for projecting a beam of light onto the optical fiber. Two optical detectors are positioned on each side of the optical fiber. As a result of the coherence and monochromaticity of the laser beam, interference patterns are created in the far field which are detected by the optical detectors. Holes contained in the optical fiber result in fewer flinges in the interference patterns created in the far field. A plurality of light sources are used in order to ensure that light passes through the entire fiber so that no blind spots exist. This is intended to ensure that light will be reflected off of holes contained at any location within the optical fiber and thus will be detected by the optical detectors. Spatial frequency spectra are generated based on the output of the light detectors and the spectra are analyzed to determine whether a hole exists in the optical fiber.
The systems disclosed in Button et al. and Bondybey et al. both perform optical detection of defects in an optical fiber before any coating layers have been applied to the optical fiber. Furthermore, both of those systems are fairly complicated in terms of hardware and/or software. For example, the Bondybey et al. patent discloses using at least one photomultiplier for detecting light scattered by defects and an electrometer-strip chart recorder for generating a scattering trace. The system disclosed in the Button et al. patent requires the use of a plurality of optical detectors for projecting light onto the fiber under investigation and a plurality of optical systems for projecting far-field interference patterns caused by reflection and refraction by the fiber onto the optical detectors. A rather complex technique, which utilizes a Fast Fourier Transform (FFT) to generate a frequency spectrum, is then performed to determine the frequency of the outer diameter component of the fiber, from which the diameter of the fiber is determined, and to determine whether the frequency spectrum of the fiber matches that of a defect-free fiber of the same diameter.
In contrast, in accordance with the present invention, a system for detecting optical defects in optical fibers is provided which utilizes the optical characteristics of the optical fiber coating to reduce the complexity of the defect detection system. Specifically, the present invention utilizes the fact that only a small difference exists between the indices of refraction of the coating surrounding the fiber and the fiber itself In accordance with the present invention, light projected onto the optical fiber coating in a direction perpendicular to the fiber is reflected at the air/coating interface on which the light initially impinges and at the air/coating interface the light impinges on as it passes through the fiber and out of the coating. At the coating/fiber interface the light is not reflected since the refractive indices of the coating and of the fiber are very close in magnitude. Therefore, when no defects exist in the fiber, two rays of light can be detected, one resulting from the first reflection at the air/coating interface and another resulting from the second reflection at the air/coating interface. However, when a defect is present in the fiber, three rays of light can be detected, the first and second reflections at the air/coating interfaces and a third reflection caused by the defect. In accordance with the present invention, it has been determined that all three of these reflections will be parallel to each other and orthogonal to the direction of the light projected onto the coating. An optical detection device detects these reflections and a signal processing device determines whether a defect exists in the fiber based on the number of reflections detected.